Prefused descaling bath constituent and method of maintaining a constant chemical composition of a bath

ABSTRACT

A prefused and solidified salt for use in forming a metalconditioning bath and for additives to existing baths. The slat is a uniform solid solution of an alkali metal hydroxide, a nitrate salt, and water.

United States Patent Detroit, Mich.

PREFUSED DESCALING BATH CONSTITUENT AND METHOD OF MAINTAINING A CONSTANT CHEMICAL COMPOSITION OF A BATH 19 Claims, No Drawings U.S. Cl 252/87, 134/2, 204/145 F, 252/181 lnt.Cl. C23b 1/06, C23g1/28 References Cited UNlTED STATES PATENTS Gilbert Ferguson. Huff Beigay.. Zaremski.. Bartek .t

Primary Examiner-John T. Goolkasian Assistant Examiner-M. E. McCamish Attorney-William N. Hogg 1501 FieldotSearch ..2s2/ss,s7, 80,84, 86, 175, 180, 181; 134/23;2o4/1so.s, 143,

134/2 134/2 X 134/2 134/2 252/86 X 134/2 X ABSTRACT: A prefused and solidified salt for use in forming a metal-conditioning bath and for additives to existing baths. The slat is a uniform solid solution of an alkali metal hydroxide, a nitrate salt, and water.

PREFUSED DESCALING BATH CONSTITUENT AND METHOD OF MAINTAINING A CONSTANT CHEMICAL COMPOSITION OF A BATH This invention relates generally to constituents for salt baths used to condition metal oxide scale at elevated temperatures, and more particularly to an improved constituent for such baths and the use thereof to maintain the chemical content of such baths relatively constant.

in the manufacture of metal shapes and metal objects and in the production of certain metals it is often necessary to heat the metal between and during processing and fabricating operations. Such heat treatments are frequently conducted in an oxidizing atmosphere which causes an oxide scale to form on the metal. ln most cases it is necessary to remove this oxide scale and there have been numerous proposals for scale removal.

Normally, the removal of the scale requires some type of pickling operation, conventionally in an acid medium. In some instances, however, this scale is of such a character that it is either difficult or impossible to remove merely by pickling in an acid or various combinations of acids; or, the time required to remove such scale solely by pickling in an acid maybe so long as to be economically undesirable.

There have been several prior art proposals for improving the scale removal procedure for different types of alloys and some of these proposals utilize a scale-conditioning treatment in salt baths at elevated temperatures followed by a pickling operation. Examples of these salt bath scale-conditioning treatments are found in U.S. Pat. Nos. 3,l2l,026 and 3,254,0l l. These patents disclose the use of scale-conditioning baths which are comprised of aqueous solutions of potassium hydroxide and potassium nitrate maintained at elevated temperatures. Basically, these patents disclose the treatment of metal oxides such as those found on titanium and stainless steel by passing the metal with the oxide thereon through the salt bath maintained at an elevated temperature for conditioning the scale and thereafter pickling the material to remove this preconditioned scale.

These baths of aqueous solutions of potassium hydroxide and potassium nitrate have worked extremely well for this scale-conditioning process as have other baths of alkali metal hydroxides-such as sodium hydroxide with nitrate additions thereto.

According to prior art practice a large tank is initially supplied with a physical mixture of the alkali metal hydroxide and the nitrate salt and water in the desired proportions and then the bath is heated to the desired operating temperature fusing and dissolving the hydroxide and nitrate in the water, and the bath then is maintained at this elevated temperature for scale conditioning. The metal to be treated is immersed and then removed from the bath, after which the conditioned scale is removed by acid pickling. This immersing and removing of the metal from the bath results in certain losses to the bath. One of the principal factors contributing to this loss is the so called dragout phenomenon. This simply means that when the metal piece is removed from the bath a certain amount of the bath tends to adhere to the surface of the piece and over a period of time this results in a loss to the bath. Other factors, such as entrapment by sludge also contributes to the loss of the bath constituents. Thus, it becomes necessary to periodically make additions to the bath of the various bath constituents in order to maintain the bath at the desired level. ln the past, these additions normally were accomplished by adding a physical mixture of granules of the potassium hydroxide and the potassium nitrate and adding the desired amount of water by some type of spray system while agitating the bath.

This technique of bath additions has several undesirable aspects and also causes certain undesirable bath characteristics. One undesirable characteristic of adding the separate ingredients is that the constituents are rather slow to dissolve in the bath, thus delaying the time which it takes them to become active inasmuch as they are not active until they are in a fused form. Another undesirable aspect is that when additions to the bath are made as separate constituents the composition of the bath tends to change, even though the additions are of the same proportions as the constituents of the bath. This normally is characterized by a decreasing percentage or proportion of the nitrate and obviously has the disadvantage of causing the bath to be nonuniform. This in turn varies its scale-conditioning characteristics which causes variations in time necessary for the treatment. This changing of constituent proportions of the bath also causes certain other undesirable aspects such as a tendency toward pitting and nonuniform scale treatment.

Another disadvantage of the addition of the constituents separately is that water must be added to a high-temperature liquid bath which can cause certain safety problems. If proper agitation is not maintained or the water is added improperly, an explosion can occur scattering the molten salts.

it is therefore a principle object of this invention to provide an improved aqueous alkali metal hydroxide-nitrate salt material for forming scale-conditioning baths and for additions to existing scale-conditioning baths.

Another object of this invention is to provide an improved scale-conditioning bath material which is suitable for additions to said baths which comprises a uniform solid solution of the desired material composition. I

Still a further more particular object of this invention is to provide an improved method of preventing or reducing the tendency toward change of constituent proportions in scaletreating baths.

These and. other objects, together with a fuller understanding of the invention may be had by reference to the following specifications and appended claims.

Briefly, the present invention contemplates the provision of a material which is adapted to form or be added to existing scale-conditioning baths of the alkali metal hydroxide-nitrate salt water type maintained at elevated temperatures. The invention is especially applicable although not limited to baths of aqueous solution of potassium hydroxide and potassium nitrate which are utilized for scale conditioning of titanium, stainless steel, super alloys and other metals and alloys having scale which is difi'lcult to remove by acid pickling alone. The invention contemplates the provision of a prefused solidified material having a uniform composition which is a solid solution of the alkali metal hydroxide and the nitrate salt with water.

More particularly, the invention will be described in relationship to the potassium hydroxide, potassium nitrate baths of the type disclosed in the U.S. Pat. Nos. 3,254,0ll and 3,l2l,026. However, it is to be understood that it is not limited to the precise compositions disclosed in these patents but is adapted for use for other scale-conditioning baths which include alkali metal hydroxide and nitrate salt combinations in aqueous solution.

Specifically, in order to produce a salt for providing or adding to a bath of the compositions shown in the said patents the desired proportions of potassium hydroxide and potassium nitrate are first determined.

The potassium hydroxide is the basic or principal constituent for the scale conversion function. The potassium nitrate is present in order to avoid certain harmful effects to certain alloys which may result from the use of potassium hydroxide alone. These surface effects include such phenomenon as hydrogen pickup by titanium alloys and a propensity toward pitting of stainless steel and other alloys. The potassium nitrate also aids in the conversion of the scale of stainless steel and certain super alloys. There should be at least 5 percent potassium nitrate to perform these functions effectively, but there should be no more than 30 percent potassium nitrate, inasmuch as concentration of more than 30 percent of potassium nitrate do not provide any additional beneficial results and indeed are detrimental in that there is a reduced amount of available potassium hydroxide to perform the basic scale converting function. Therefore, the potassium hydroxide percentage should be between 5 and 30 percent. In

the finished salt or, the salt as it is to be-used in the descaling temperatures in the bath, there should be at least 5 percent water in order to allow the bath to be liquid at temperatures below 500 F. Without at least 5 percent water the salt in effect becomes a nonaqueous solution and must be heated well above 600 F. for fusion. The 5 percent water provides for the aqueous-type solution and fusion at temperatures below 500 F. There should not be any more than 20 percent water in the final salt as it is to be used, inasmuch as this tends to make the salt damp and sticky which is undesirable which will be explained in detail hereinafter. One method of manufacturing a flaked product with an approximate composition of about 75 percent potassium hydroxide, about I 15 percent potassium nitrate and about percent water and normal impurities is as follows.

A solution of 55 percent water and 45 percent potassium hydroxide is placed in a reaction vessel and the heating started. As the water evaporates more of the original material is added, and the heating continues until the temperature of the vessel reaches about 645 F. The material and vessel are then cooled to about 500 F. at which time the required amount of potassium nitrate is added, this being in proportion of parts potassium nitrate to 75 parts of potassium hydroxide by weight. The heat is then adjusted to maintain a temperature of about 465 F. until all of the potassium nitrate has gone into solution. At this point, the temperature is reduced to 435 F. at which temperature the material is flaked in conventional flaking equipment.

It is to be understood that the amount of water which will be present is essentially a function of time and temperature which when varied will vary the final amount of water present.

As was indicated above, it is desirable to keep the water content below percent, since if there is more than 20 percent water the product will not be a relatively dry, free-flowing flake, but rather will become damp and adhere together. This makes the addition of the product to the bath extremely difficult and the additional water serves no function as in most cases the percentage of water in the bath is to be less than 20 percent. Also, it is believed that any water in excess of 20 percent is present as moisture adhering to the flakes and as such would constitute an addition of water to a bath if the salt were added to the existing bath. This, as explained above, can constitute a safety hazard if proper agitation is not present. The most usual range for water content based on manufacturing considerations and normal bath requirements is from 5 to 10 percent.

As was explained above, one of the principal benefits from a prefused salt of this invention, as opposed to the physical mixtures as previously employed, is derived when it is used for additions to existing scale-conditioning salt baths. It has been found that when the prefused and solidified product of this invention is added to existing commercial salt bath installations, whether formed initially from the prefused product or from a physical mixture, there is a substantially reduced tendency toward a change of bath composition as opposed to when additions are made by adding physical mixtures of potassium nitrate and potassium hydroxide. in the past, this has been a problem which required frequent chemical analysis of the descaling bath with attendant selective additions of potassium nitrate alone, as well as additions of potassium nitrate and potassium hydroxide to maintain the level of the bath. The reason that the prefused and solidified salt of this invention is so effective in preventing or reducing this tendency is not completely understood. However. it is believed that the following is at least a partial explanation of the superiority.

First, a uniform solid solution of a salt according to this invention containing approximately 75 percent potassium hydroxide, 15 percent potassium nitrate and 10 percent water has a melting point of approximately 284 F. with a boiling point well over 500 F. However, potassium nitrate has a melting point of 630 F potassium hydroxide has a melting point of 680 F. and water has a boiling point of 212 F. Thus, it is apparent that the potassium hydroxide, potassium nitrate and water form a solution which is somewhat eutectic in nature. Hence, when a solid particle of the prefused and solidified salt is added to an operating bath, which operating temperature is normally between 400 and 500 F the material immediately encounters a solution temperature which is above the melting point of each particular particle. Thus, the uniform solid solution of each particle merely melts and changes from a solid solution to a liquid solution of uniform composition with no chemical action or reaction taking place. However, in the case of solid additions of the potassium hydroxide and potassium nitrate as separate particles, each particle encounters a bath having a temperature which is below its melting point. These particles therefore will remain in the solid state until the nitrate and hydroxide particles encounter each other in sufficient proportions to go into the liquid solution at this reduced temperature.

in a new bath, which is uncontaminated and has no foreign material suspended therein this encountering process will eventually take place and a complete dissolving and dispersion of the material will take place to form a uniform bath of the desired compositional limits based on the composition of these materials added. But, in an existing bath that is in use for descaling, this is not the case.

In an existing bath that is in use, there are present contaminates in the form of carbonates and particles of scale and other foreign objects which are maintained in suspension or intermixed in the bath due to bath agitation. These solid particles form a sludge which precipitates at the bottom of the bath. When the additions are made as solid separate particles of nitrate and hydroxide a portion of these fine particles get dispersed within the bath, but before they go into solution they encounter these contaminating particles and a certain amount of the nitrate and hydroxide additives are carried into the sludge by these contaminating particles and thus never actually go into the solution in the bath. Since the nitrate is the constituent that is normally lower than would be expected from the added proportions, it is believed that there is more of a propensity for the nitrate particles to collect on the contaminates and go into the sludge than there is for the hydroxide particles.

Thus, because of the particulate impurities contained in an operating bath there is more of a propensity for change of composition when additions are made with the additives in separate particulate form, as opposed to the prefused uniform salt additive.

Also, it is believed that there is some change in proportions due to the particles of the separately added constituents adhering to the surface of the material as it is passed through the bath and being withdrawn therefrom as additional dragout over and above the normal dragout of the salt caused by the material being withdrawn from the bath. it is theorized that there is a greater propensity for the nitrate particles to adhere to the surface of the article being treated than for the hydroxide particles to so adhere.

Thus, the use of a prefused solid solution product according to this invention provides a particle of uniform composition which goes into solution merely by a phase change from solid to liquid and thus does not provide separate particles of the separate constituents which can be prevented from going into solution during this dissolving process. Indeed, in practice it has been found that when the prefused product according to this invention is used, the bath maintains such a constant composition balance that the frequent analyses of the bath, which in the past had been required, can be eliminated, or drastically reduced in number.

Another disadvantage of making the additions of the constituents as separate materials rather than as a uniform prefused salt is that there is a tendency toward pitting and uneven or nonuniform scale treatment when the additions are as such separate particles. lt is believed that this is due to the fact that while the separate additives are still in particulate form and just dissolving into the bath, there will be a nonuniforrnity of concentration of the bath which condition will exist until virtually all of the material has gone into the solution that will go in and is mixed throughout the bath. This nonuniformity of concentration during the dissolving period of the additives is believed to contribute to this nonuniform action or reaction of the bath with the scale. Thus, in the areas of high concentration of nitrate the action will be reduced and the action of the nitrate increased thereby tending to use up the nitrate faster; whereas, in the areas of high concentration of the hydroxide there may be some tendency toward an increased action thereof with possible attendant result of pitting or surface attach 0f the metal.

It has further been found that it takes substantially a longer period for the additions made as separate materials to go into solution in the bath, than for a prefused and solidified uniform solid solution of the constituents according to this invention. This increased rate of going into solution has been observed on commercial baths and because of this the additions made to the bath are more quickly available when made in the prefused uniform state than when in the prior art form of mixed constituents, as the bath can function as a scale conditioner only when it is in the liquid form. Therefore, the sooner the particles are in liquid form the sooner they are able to function for their intended purpose.

it has also been observed'that the prefused flakes tend to coalesce when they are added to the bath and this mass then melts from the outside. This is in contradistinction to the prior art mixtures wherein many separate chunks float about contributing to the previously described drawbacks of nonuniformity of composition and composition change.

This quicker melting of the prefused product according to this invention as compared to the prior art physical mixtures of constituents also is of benefit in new baths in that the bath is ready sooner after the material is supplied and heating started. This increased rate was observed on a laboratory scale wherein two identical pots were provided and identically heated. Ten pounds of a prefused salt having 75% KOH, KNO and 10% H O was added to one pot and a mixture of 83.3% commercial potassium hydroxide (which contains 10% water) 15.0% KNO and 1.7% H 0 was added to the other pot (this total composition mixed in the second pot equalled the compositional constituents of the solid solution prefused and solidified salt in the first pot). Heat was supplied to each of the pots at identical rates and after 36 minutes the prefused mix was completely liquid. it wasn't until after 44 minutes that the physical mixture of constituents in the second pot became completely liquid. It was also noted that even at this point in time, i.e., after 44 minutes the liquid was not completely clear, as was the case of the prefused salt, but actually it had a cloudy or milky appearance. This would indicate the presence of undissolved particles held in suspension within the liquid. Although the nature of these particles could not be precisely determined, it is believed that they were principally particles of the nitrate. Hence, if descaling is started before the liquid clears up which in the case of the laboratory sample was not until several hours later, then the foreign particles introduced into the bath would tend to collect with these suspended particles that had not gone into the solution and precipitate them out in the sludge, as explained above.

Thus, it can be seen that a prefused and solidified solid solution of an alkali metal hydroxide and nitrate salt with water provides a superior material both for forming a new bath for treating metal oxide scale and also it provides a superior constituent for additions to existing salt baths, which in both cases the improved results are obtained from the use of such a prefused solid solution.

Although several embodiments of this invention have been shown and described, various adaptions and modifications may be made without departing from the scope of the appended claims.

We claim:

ing at elevated temperatures, said material having been solidified from a liquid sufficient] rapid] to form a uniform solid solution comprising an alka 1 metal ydroxide, a nitrate salt, and water.

2. The material of claim 1 wherein the alkali metal hydroxide is selected from the group consisting of sodium hydroxide and potassium hydroxide.

3. The material of claim 2 wherein the nitrate salt is an alkali metal nitrate.

4. The material of claim 3 wherein the alkali metal nitrate is selected from the group consisting of sodium nitrate and potassium nitrate.

5. The material of claim 4 wherein the alkali metal nitrate is potassium nitrate.

6. The material of claim 5 wherein there is from 60 to percent by weight of potassium hydroxide, 5 to 30 percent by weight of potassium nitrate and 5 to 20 percent by weight of water.

7. The material of claim 6 wherein there is no more than l0 percent water.

8. The invention as defined in claim 1 wherein the material is in the form of flakes.

9. A method of maintaining a preselected chemical content in a scale-conditioning bath which is an aqueous solution of an alkali metal hydroxide and a nitrate salt maintained at elevated temperatures, and wherein periodic additions of the constituents is required, which method comprises, providing a fused and solidified material which is a uniform solid solution of said alkali metal hydroxide and said nitrate salt in the desired relative proportions and water and adding quantities of said material to said bath as required.

10. The method of claim 9 wherein the bath is a potassium hydroxide, potassium nitrate, water bath and wherein the material to be added has the potassium hydroxide and potassium nitrate in substantially the same proportions of the bath, and has from 5 to 20 percent water.

11. The method of claim 10 wherein the material to be added has from 6 to 90 percent potassium hydroxide, from 5 to 30 percent potassium nitrate, and no more than 10 percent water.

12. The invention as defined in claim 9 wherein the material is in the form of flakes.

- l3. A method of forming a material adapted for conditioning metal scale which material is solid at ambient temperature comprising the steps of heating an alkali metal hydroxide, a nitrate salt and water to a temperature sufficiently high and for a sufficient time to provide a uniform solution of said hydroxide, said nitrate and water, which is liquid at said temperature and solid at ambient temperature, and cooling said solution to ambient temperature sufficiently rapidly to form a uniform solid solution of said constituents.

14. The method of claim 13 wherein said hydroxide is provided in an aqueous solution and heated to an elevated temperature, and wherein said nitrate salt is then added.

15. The method of claim 14 wherein said hydroxide is potassium hydroxide and said nitrate is potassium nitrate.

16. The method of claim 15 wherein the potassium hydroxide solution is heated to about 645 F. at which temperature the potassium nitrate is added.

17. The method of claim 16 wherein the temperature is reduced to about 465 F. after the potassium nitrate is added and held at said temperature until the nitrate has gone into solution.

18. The method of claim 17 wherein the temperature is reduced to 435 F. after the nitrate has gone into solution and the material flaked at this temperature.

19. The invention as defined in claim 13 wherein the material is flaked during cooling. 

2. The material of claim 1 wherein the alkali metal hydroxide is selected from the group consisting of sodium hydroxide and potassium hydroxide.
 3. The material of claim 2 wherein the nitrate salt is an alkali metal nitrate.
 4. The material of claim 3 wherein the alkali metal nitrate is selected from the group consisting of sodium nitrate and potassium nitrate.
 5. The material of claim 4 wherein the alkali metal nitrate is potassium nitrate.
 6. The material of claim 5 wherein there is from 60 to 90 percent by weight of potassium hydroxide, 5 to 30 percent by weight of potassium nitrate and 5 to 20 percent by weight of water.
 7. The material of claim 6 wherein there is no more than 10 percent water.
 8. The invention as defined in claim 1 wherein the material is in the form of flakes.
 9. A method of maintaining a preselected chemical content in a scale-conditioning bath which is an aqueous solution of an alkali metal hydroxide and a nitrate salt maintained at elevated temperatures, and wherein periodic additions of the constituents is required, which method comprises, providing a fused and solidified material which is a uniform solid solution of said alkali metal hydroxide and said nitrate salt in the desired relative proportions and water and adding quantities of said material to said bath as required.
 10. The method of claim 9 wherein the bath Is a potassium hydroxide, potassium nitrate, water bath and wherein the material to be added has the potassium hydroxide and potassium nitrate in substantially the same proportions of the bath, and has from 5 to 20 percent water.
 11. The method of claim 10 wherein the material to be added has from 6 to 90 percent potassium hydroxide, from 5 to 30 percent potassium nitrate, and no more than 10 percent water.
 12. The invention as defined in claim 9 wherein the material is in the form of flakes.
 13. A method of forming a material adapted for conditioning metal scale which material is solid at ambient temperature comprising the steps of heating an alkali metal hydroxide, a nitrate salt and water to a temperature sufficiently high and for a sufficient time to provide a uniform solution of said hydroxide, said nitrate and water, which is liquid at said temperature and solid at ambient temperature, and cooling said solution to ambient temperature sufficiently rapidly to form a uniform solid solution of said constituents.
 14. The method of claim 13 wherein said hydroxide is provided in an aqueous solution and heated to an elevated temperature, and wherein said nitrate salt is then added.
 15. The method of claim 14 wherein said hydroxide is potassium hydroxide and said nitrate is potassium nitrate.
 16. The method of claim 15 wherein the potassium hydroxide solution is heated to about 645* F. at which temperature the potassium nitrate is added.
 17. The method of claim 16 wherein the temperature is reduced to about 465* F. after the potassium nitrate is added and held at said temperature until the nitrate has gone into solution.
 18. The method of claim 17 wherein the temperature is reduced to 435* F. after the nitrate has gone into solution and the material flaked at this temperature.
 19. The invention as defined in claim 13 wherein the material is flaked during cooling. 